Integrated circuits (ICs) are ubiquitous in the modem electronic age, found in a wide variety of applications and products, including household appliances, consumer electronics, communications systems, automotive systems, etc. As dependence on ICs has increased, the reliability of these devices has become increasingly critical. Thus, an important objective of IC designers and manufacturers is to produce an electronic component and/or system having a high degree of reliability, an essential attribute for gauging the quality of electronic components and systems. Moreover, both manufacturers and customers of ICs strive to define and predict such reliability in a cost-effective manner.
ICs are subject to what is often referred to as infant mortality failures. Infant mortality failures are those failures that are associated with a wear-out mechanism causing device failure within a relatively short period of operating time, far below a predicted mean time before failure (MTBF) for the device (e.g., within six months of operation). Dynamic random access memory (DRAM) is particularly prone to infant mortality failures which may be attributable to, for instance, storage cell capacitor dielectric failure.
ICs containing DRAM, for example, commodity DRAM ICs and logic ICs with embedded DRAM (eDRAM), are also subject to variable-hold-time (VHT) failures. Hold time is typically defined as an amount of time that a DRAM will retain data without being refreshed. Minimum hold time is usually specified by a DRAM manufacturer and is typically somewhere between 10 microseconds (μs) and 10 milliseconds (ms). A hold time failure can occur when one or more storage cells (in one or more respective memory cells) leaks charge so fast that it loses data before the next scheduled refresh. A hold time failure is typically caused by a fault in the cell capacitor dielectric which leaks charge from the cell storage capacitor. This type of hold time failure is typically a hard failure. A hard failure is one that reoccurs repeatedly.
VHT failures are associated with changes, over time, of the hold time of a cell, such as, for example, variations in hold time wherein the hold time changes from an acceptable or passing value, to a failing value, and back to a passing value. A VHT failure is essentially a hold time failure that occurs intermittently. It may occur so infrequently that it may not be caught at a manufacturer IC testing stage, but may fail intermittently in a customer application. Failures that occur intermittently are often termed soft failures. Soft failures are a troublesome source of IC quality issues.
In order to segregate ICs having VHT failures from ICs that do not, it is typically necessary to perform continuous testing for extended periods of time and under various operational conditions (e.g., supply voltage, temperature, clock speed, etc.), also known as burn-in testing. In this way, many intermittent failures can be detected and yield can be advantageously improved by eliminating ICs exhibiting such intermittent failures. However, the more complex and rigorous the post-manufacturing testing becomes, the higher the cost of the testing, which is undesirable.